Oligonucleotides and oligonucleotide analogs conjugated with functional groups have unusual properties which make them useful as hybridization probes and antisense reagents (1-3). These functional groups can include intercalators such as acridine, reporter groups such as fluorescent or spin labels, immunoreactive groups such as biotin, or chemically reactive groups such alkylating groups, psoralen or metal chelators. See, for example, U.S. application Ser. No. 06/924,234 filed Oct. 28, 1986, abandoned the disclosure of which is incorporated herein by reference.
The functional groups are usually conjugated with the oligomer via a linker arm of variable chain length. The linker arm in turn may be attached to the 3' or 5' end of the oligomer, to the internucleotide linkage of the oligomer or to a modified base located at a specific site within the oligomer.
A variety of methods are available for introducing the linker into the oligomer. Solid phase supports have been developed which incorporate a linker as part of the support (4-9). These allow synthesis of oligomers with linkers usually consisting of an aminoalkyl chain at the 3' end of the molecule. Linkers may be introduced at the 3'- or 5'-position by derivatization of terminal phosphate groups with diaminoalkanes or amino acids (10-14). H-phosphonate chemistry has been used to introduce linkers at specific internucleotide linkages within the oligomer (15-18). Protected phosphoramidite reagents and nucleoside phosphoramidite synthons have been developed which allow incorporation of linkers or linker-modified nucleosides during the course of oligonucleotide synthesis (19-23).
It is also known that aminoalkyl groups may be selectively incorporated at the N.sup.4 -position of cytosine by means of a bisulfite-catalyzed transamination reaction (24). This bisulfite-catalyzed transamination reaction is relied upon to provide the novel oligonucleotides of the invention.